1. Field of the Invention
The present invention relates to a radiation detector and a radiation imaging apparatus having a function of detecting radiation including X-ray, ã-ray, light and the like and used in the medical, industrial, and nuclear fields. More specifically, the present invention relates to the art wherein a radiation-sensitive detection layer is formed of a semiconductor which is formed by polycrystals.
2. Description of the Related Art
As conventionally known radiation detectors, those using a monocrystal material of CdTe (cadmium telluride), CdZnTe (cadmium zinc telluride) for a radiation-sensitive detection layer can be recited. Since these radiation detectors have wide gap and comprise heavy atoms, they are operable at room temperature and have high sensitivity. However, it is extremely difficult to make monocrystals grow over a large area for surface imaging, and an attempt has been made to form a surface detector by patching monocrystal members of small area together in tile-like manner. However, for obtaining a large area of several centimeters square which is required for industrial or medical imaging apparatus, the material cost of monocrystal members is extraordinarily high and treatment of the joints is complicated.
On the other hand, polycrystal films of CdTe (cadmium telluride) and CdZnTe (cadmium zinc telluride) formed by CVD method, PVD method and the like include many crystal grain boundaries, and experiments carried out by the present inventors have revealed that the polycrystal films are inferior in electrical and radiation detecting characteristics to monocrystals if used as deposition. That is, in the case of using as a radiation detector of X-ray area, the detection layer needs a thickness of several hundreds of micrometers for absorbing X rays. When a bias is applied to a detection layer of polycrystal having such a large thickness so as to collect signal charges by X-ray irradiation, the generated charges are trapped by grain boundaries of polycrystal and the like, causing significant decrease in sensitivity and responsibility.
By the way, it is also known that, as is the case of a solar battery having a conversion layer of polycrystal thin film (several tens of micrometers) of CdTe (cadmium telluride), by applying a CdCl2 (cadmium chloride) solution of about 0.3 M on the surface of the polycrystal thin film and executing heat treatment for about 25 minutes under the atmosphere of about 420° C., it is possible to improve the light converting efficiency (JJAP, Vol. 36 (1997) Part 1, No, pp6304-6305). This would attribute to the fact that CdCl2 (cadmium chloride) infiltrate into the film and protects the grain boundaries of the polycrystal thin film.
However, the examples of conventional arts have the following problems.
That is, even if the conventional approach is applied to the radiation detector in which the detection layer has a thickness of as large as about 100 times of thickness of solar battery, CdCl2 (cadmium chloride) does not sufficiently infiltrate into the film, so that satisfactory effect cannot be obtained. In addition, if such an approach is applied to a substrate having a large area, it is impossible to achieve sufficient in-plane uniformity by coating.